1. Field of the Invention
This invention relates to a wheelchair, and more particularly to a wheelchair suspension.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a conventional suspension disclosed in U.S. Patent Application Publication No. 2004/0060748 A1 is incorporated in a wheelchair 100, and has two side frame assemblies 1, each of which includes a front caster assembly 11, a rear caster assembly 12, a linkage 13, a drive wheel 14, a driving unit 15 and a return spring unit 16.
The front caster assembly 11 includes a front mainframe 112, a front caster frame 113 extending downwardly from a front end of the front mainframe 112, a drive wheel frame 114 extending downwardly from a rear end of the front mainframe 112, and a front caster 115 disposed pivotally on a lower end of the front caster frame 113.
The rear caster assembly 12 includes a rear mainframe 122, a front connecting frame 123 extending downwardly from a front end of the rear mainframe 122, a rear caster frame 124 extending downwardly from a rear end of the rear mainframe 122, and a rear caster 125 disposed pivotally on a lower end of the rear caster frame 124. The rear mainframe 122 cooperates with the front mainframe 112 to support a seat 300.
The linkage 13 has two ends connected respectively and pivotally to the front caster frame 113 and the front connecting frame 123.
The drive wheel 14 has a diameter greater than those of the front and rear casters 115, 125, and is disposed pivotally on the corresponding drive wheel frame 114.
The driving unit 15 includes an electric motor 151 installed on the corresponding drive wheel frame 114 and operable to rotate the corresponding drive wheel 14.
The return spring unit 16 includes a front spring 162 interconnecting the corresponding front mainframe 112 and the corresponding front connecting frame 123, and a rear spring 162′ interconnecting the corresponding drive wheel frame 114 and the corresponding rear mainframe 122.
Referring to FIGS. 2 and 3, because of the presence of the front and rear springs 162, 162′, the drive wheel 14 and the front and rear casters 115, 125 can be brought into contact with a horizontal surface simultaneously.
Referring to FIG. 4, when the front caster 115 climbs over an obstacle 200 in the wheelchair's path, such as a stair, the front and rear springs 162, 162′ are stretched so as to allow the front caster 115 to rise relative to the drive wheel 14 and the rear caster 125. After the rear caster 125 surmounts the obstacle 200, the front and rear springs 162, 162′ return to their original shapes so as to allow the front caster 115 to contact a top surface of the obstacle 200.
To test the wheelchair 100, the following parameters were used: a radius of each of the front and rear casters 115, 125 of 75 mm, a distance between the axles of the front casters 115 and the rear casters 125 of 668 mm, a radius of each of the drive wheels 14 of 130 mm, a person sitting on the wheelchair 100 weighing 70 kilograms, and a total weight of the remaining portion of the wheelchair 100 supported by the front and rear casters 115, 125 and the drive wheels 14 of 74 kilograms. When the front casters 115 surmount different obstacles 200, corresponding inclination angle changes of the seat 300 of the wheelchair 100 are as outlined in the following chart:
Obstacle Height (mm)102030405060Angle Change (degrees)1.262.543.835.136.447.77
In the case where the height of the obstacle 200 is 50 mm, when the front casters 115 contact the obstacle 200, the load ratios of the front casters 115, the rear casters 125 and the drive wheels 14 are respectively 18.9%, 11.6% and 69.5%; after the front casters 115 surmount the obstacle 200 and before the drive wheels 14 contact the obstacle 200, the load ratios of the front casters 115, the rear casters 125 and the drive wheels 14 are respectively 16.9%, 26.8% and 56.3%; after the front casters 115 surmount the obstacle 200 and when the drive wheels 14 contact the obstacle 200, the load ratios of the front casters 115, the rear casters 125 and the drive wheels 14 are respectively 21.4%, 34.2% and 44.4%; after the drive wheels 14 surmount the obstacle 200 and before the rear casters 125 contact the obstacle 200, the load ratios of the front casters 115, the rear casters 125 and the drive wheels 14 are respectively 9.9%, 24.6% and 65.5%; after the drive wheels 14 surmount the obstacle 200 and when the rear casters 125 contact the obstacle 200, the load ratios of the front casters 115, the rear casters 125 and the drive wheels 14 are respectively 0.3%, 23.5% and 76.2%; and after the rear casters 125 surmount the obstacle 200, the load ratios of the front casters 115, the rear casters 125 and the drive wheels 14 are respectively 15.4%, 21.9% and 62.7%.
As indicated by the test data in the chart that, even when the height of the obstacle 200 is 60 mm, the inclination angle change of the seat is only 7.77°. This poses no safety risk and provides no feeling of discomfort to the person sitting on the wheelchair 100.
In the case where the obstacle 200 is 50 mm high, however, after the drive wheels 14 surmount the obstacle 200, and when the rear casters 125 contact the obstacle 200, the load ratio of the front casters 115 is only 0.3%. Since load ratio is proportional to the anti-tip and supporting ability of a wheel or caster, the wheelchair 100 tends to tip or become unstable after the drive wheels 14 surmount the obstacle 200, and when the rear casters 125 contact the obstacle 200.